Preparation method for hollow molybdate composite microspheres and their application

ABSTRACT

A method of preparing hollow molybdate composite microspheres includes steps of: (1) dissolving 1-4 mmol of MCl2 in 20 ml of water to obtain a solution A and dissolving 1-4 mmol. of molybdic acid in 20 ml of water to obtain a solution B, followed by mixing the solution A and the solution B, in which M is Co, Ni, or Cu; (2) dissolving 10-40 mmol of urea in 40 ml of water, adding the mixed solution of step (1) and stirring uniformly; (3) placing the mixed solution of step (2) into a reaction vessel and reacting at 120-160° C. for 6-12 hours; (4) suction filtrating and water washing, followed by drying in a vacuum oven at 40-60° C.; (5) calcination at 350-500° C. for 2-4 hours in a Muffle furnace.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of PCT application No.PCT/CN2018/117564, filed on Nov. 27, 2018, which designates UnitedStates and claims priority of China Patent Application No.201711266864.8, filed on Dec. 12, 2017 which is incorporated herein byreference.

BACKGROUND OF THE INVENTION 1. Technical Field

The invention relates to the technical field of composite preparation,in particular to a method for preparing hollow molybdate compositemicrospheres and application thereof as a catalyst for ammonia boranehydrolysis to produce hydrogen.

2. Description of Related Art

A molybdate is composed of one or more cations and (MoO₄)²⁻ withdifferent properties, for example, lanthanum molybdate and europiummolybdate can be fluorescent powder materials; the cobalt, nickel andcopper molybdate can act as catalysts with excellent catalyticperformance. The molybdates with different compositions can also bewidely applied in the fields of optics, electricity, catalysis andmedical treatment, so the molybdate is an important member of inorganicfunctional materials.

In recent years, the preparation method for molybdate nanomaterials hasbeen a research hotspot. It needs higher temperature and cannotguarantee the crystal with better morphology in the traditionalhigh-temperature solid-state method. Therefore, some low-temperaturesynthesis methods, such as hydrothermal synthesis, template method,microemulsion method and precipitation method, have emerged. HollowCdMoO₄ microspheres have been synthesized by precipitation in aqueoussolution at room temperature (L. Zhen et al. High photocatalyticactivity and photoluminescence property of hollow CdMoO₄ microspheres.Scripta Materials, 2008, 58, 461-464). Although the soluble inorganicsalt NaCl added in the reaction process does not participate in thereaction, the precipitation speed of CdMoO₄ is influenced when NaCl isserved as an addition agent; however, according to the preparationmethod, the suspension liquid is required to stand for 5 days, theperiod is long, and industrial production is not facilitated. CoMoO₄nanorods are synthesized by a solvothermal synthesis method, a precursoris prepared by using a mixed solution of ultrapure water, ethanol andethylene glycol as a solvent in a reaction process, and the CoMoO₄nanorods are obtained by calcining under an argon atmosphere (LiangjunWang et al. Synthesis of porous CoMoO₄ nanorods as a bifunctionalcatalyst for a Li—O₂ battery and superior anode for a Li-ion battery,Nanoscale, 2017, 9, 3898-3904). The established method is relativelysimple, but it uses organic solvent and rare gas, higher costs. CoMoO₄nanoplatelets are synthesized by a two-step hydrothermal method usinggraphene as a template, and argon as a protective gas in the calcinationstage, still making the preparation cost very high (Xiaoqin Yan et al.3D architecture of a graphene/CoMoO₄ composite for asymmetricsupercapacitors usable at variable temperatures. J. Colloid InterfaceSci., 2017, 493, 42-50). The morphology of the synthesized products ofcobalt molybdate and copper molybdate mostly are nanoplatelets ornano-particles, and hollow microspheres are not reported. The hollowspheres have high effective contact area and porosity, which is apromotion for the activity of catalysts.

BRIEF SUMMARY OF THE INVENTION

The present invention aims to solve the key problems of high preparationcost, uncontrollable morphology and the like in the preparation process.The present invention provides a method for preparing multi-molybdatehollow microspheres, in which a multi-molybdate hollow microspherestructure is successfully synthesized by using urea as a precipitant anda hydrothermal synthesis method; the synthesis method provides technicalsupport for systematically studying the structure-activity relationshipbetween the microstructure and the performance of the multi-molybdatenanomaterial, and also provides an important step for promoting low-costand large-scale production of the material.

In order to solve the above technical problems, the technical solutionadopted by the invention is as follows: a method for preparing hollowmolybdate composite microspheres includes steps of:

(1) dissolving 1-4 mmol of MCl₂ in 20 ml of water to obtain a solutionA, and dissolving 1-4 mmol of molybdic acid in 20 ml of water to obtaina solution B, followed by mixing the solution A and the solution B,wherein M is one of Co, Ni, or Cu;(2) dissolving 10-40 mmol of urea in 40 ml of water, adding the mixedsolution of step (1) and stirring uniformly;(3) transferring the mixed solution of step (2) into a reaction vesseland reacting at 120-160° C. for 6-12 hours;(4) carrying out suction filtration and water washing, followed bydrying in a vacuum oven at 40-60° C.; and(5) calcining at 350-500° C. for 2˜4 hours in a muffle furnace.

Preferably, the ratio of the total mass of soluble nickel salt, cobaltsalt and copper salt to the mass of molybdic acid in the step (1) is1:1.

Preferably, in the step (2), the stirring time is 0.5-1 h.

Preferably, in the step (3), the temperature in the vacuum oven is40-60° C.

In summary, the technical solution of the invention has the followingbeneficial effects. 1) The unique nano hollow sphere prepared by theinvention has higher effective contact area and porosity; 2) the processis simple, and the raw materials are cheap and easy to obtain; and 3)the preparation process does not require a surfactant as astructure-directing agent for morphology control.

Due to the characteristics of microstructure, the synthesizedmulti-molybdate hollow microspheres are expected to be highly activecatalysts, for example, exhibiting superior catalytic activity forammonia borane hydrolysis to produce hydrogen.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The foregoing and other exemplary purposes, aspects and advantages ofthe present invention will be better understood in principle from thefollowing detailed description of one or more exemplary embodiments ofthe invention with reference to the drawings, in which:

FIG. 1 is an SEM image of Co_(0.8)Cu_(0.2)MoO₄ prepared according to thepresent invention.

FIG. 2 is a TEM image of Co_(0.8)Cu_(0.2)MoO₄ prepared according to thepresent invention.

FIG. 3 is a BET test curve of Co_(0.8)Cu_(0.2)MoO₄ prepared according tothe present invention.

FIG. 4 is an XRD test curve of Co_(0.8)Cu_(0.2)MoO₄ prepared accordingto the present invention.

FIG. 5 is a catalytic hydrogen production test curve ofCo_(0.8)Cu_(0.2)MoO₄ prepared according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The invention will now be described in detail through severalembodiments with reference to the accompanying drawings.

First Embodiment

1.1 mmol CuCl₂ was dissolved in 20 mL water to obtain a solution A; 1mmol molybdic acid was then dissolved in 20 mL water to obtain asolution B; and then the solution A and the solution B were mixed toobtain a mixed solution C.

2. 10 mmol urea was dissolved in 40 mL water, and the solution C abovewas added; the obtained solution was stirred for 30 min, thentransferred to a reaction vessel and reacted at 160° C. for 8 h, carriedout with suction filtration and washing, and dried in a vacuum oven at40° C., and calcined in a muffle furnace at 500° C. for 2 h; the samplecomposition was CuMoO₄.

Second Embodiment

1. x mmol CuCl₂, y mmol NiCl₂ and (1-x-y) mmol CoCl₂ were dissolved in20 mL water to obtain a solution A; 2 mmol molybdic acid was thendissolved in 20 mL water to obtain a solution B; and the two solutionswere mixed to obtain a mixed solution C.

2. 20 mmol urea was dissolved in 40 mL water, and the solution C abovewas added; the obtained solution was stirred for 30 min, thentransferred to a reaction vessel and reacted at 120° C. for 12 h,carried out with suction filtration and washing, and dried in a vacuumoven at 60° C., and calcined in a muffle furnace at 500° C. for 2 h; thesample composition was Cu_(x)Co_(y)Ni_(1-x-y)MoO₄.

Third Embodiment

1. x mmol CuCl₂, y mmol NiCl₂ and (1-x-y) mmol CoCl₂ were dissolved in20 mL water to obtain a solution A; 2 mmol molybdic acid was thendissolved in 20 mL water to obtain a solution B; and the two solutionswere mixed to obtain a mixed solution C.

2. 30 mmol urea was dissolved in 40 mL water, and the solution C abovewas added; the obtained solution was stirred for 30 min, thentransferred to a reaction vessel and reacted at 160° C. for 8 h, carriedout with suction filtration and washing, and dried in a vacuum oven at40° C., and calcined in a muffle furnace at 350° C. for 2 h; the samplecomposition was Cu_(x)Co_(y)Ni_(1-x-y)MoO₄.

Fourth Embodiment

1. x mmol CuCl₂, y mmol NiCl₂ and (1-x-y) mmol CoCl₂ were dissolved in20 mL water to obtain a solution A; 2 mmol molybdic acid was thendissolved in 20 mL water to obtain a solution B; and the two solutionswere mixed to obtain a mixed solution C.

2. 40 mmol urea was dissolved in 40 mL water, and the solution C abovewas added; the obtained solution was stirred for 30 min, thentransferred to a reaction vessel and reacted at 160° C. for 12 h,carried out with suction filtration and washing, and dried in a vacuumoven at 40° C., and calcined in a muffle furnace at 500° C. for 4 h; thesample composition was Cu_(x)Co_(y)Ni_(1-x-y)MoO₄.

Fifth Embodiment

1. x mmol CuCl₂, y mmol NiCl₂ and (1-x-y) mmol CoCl₂ were dissolved in20 mL water to obtain a solution A; 4 mmol molybdic acid was thendissolved in 20 mL water to obtain a solution B; and the two solutionswere mixed to obtain a mixed solution C.

2. 40 mmol urea was dissolved in 40 mL water, and the solution C abovewas added; the obtained solution was stirred for 1 h, then transferredto a reaction vessel and reacted at 160° C. for 12 h, carried out withsuction filtration and washing, and dried in a vacuum oven at 60° C.,and calcined in a muffle furnace at 500° C. for 4 h; the samplecomposition was Cu_(x)Co_(y)Ni_(1-x-y)MoO₄.

1. SEM Analysis

FIG. 1 is an SEM image of Co_(0.8)Cu_(0.2)MoO₄ prepared according to thepresent invention. As can be seen from the scan diagram, the morphologyof Co_(0.8)Cu_(0.2)MoO₄ obtained by hydrothermal synthesis is nanowireswith a diameter of about 0.5-0.8 μm.

2. TEM Test

For the TEM images of Co_(0.8)Cu_(0.2)MoO₄ prepared according to thepresent invention, the catalyst performance of the hollow microspherescan be further confirmed from the projection view.

3. BET Test

FIG. 3 is a nitrogen adsorption and desorption isotherm curve ofCo_(0.8)Cu_(0.2)MoO₄ prepared according to the present invention, havinga specific surface area of 30.01 m²/g.

4. XRD

FIG. 4 is an XRD test of Co_(0.8)Cu_(0.2)MoO₄ prepared according to thepresent invention; the characteristic peaks of corresponding crystalplanes of CuMoO₄ and CoMoO₄ are marked in the figure.

5. Test of Catalytic Performance for Hydrogen Production

FIG. 5 is a performance test of Co_(0.8)Cu_(0.2)MoO₄ prepared accordingto the present invention as a catalyst for ammonia borane hydrolysis toproduce hydrogen, the amount of NH₃BH₃ is 3 mmol, NaOH is 20 mmol, andthe catalyst is 10 mg. The test showed that that it produced 56 mLhydrogen per minute by taking Co_(0.8)Cu_(0.2)MoO₄ as a catalyst at 25°C.

While the invention has been described in terms of several exemplaryembodiments, those skilled on the art will recognize that the inventioncan be practiced with modification within the spirit and scope of theappended claims. In addition, it is noted that, the Applicant's intentis to encompass equivalents of all claim elements, even if amended laterduring prosecution.

What is claimed is:
 1. A method for preparing hollow molybdate compositemicrospheres, comprising steps of: (1) dissolving 1-4 mmol of MCl₂ in 20ml of water to obtain a solution A, and dissolving 1-4 mmol of molybdicacid in 20 ml of water to obtain a solution B, followed by mixing thesolution A and the solution B, in which M being one of Co, Ni, or Cu;(2) dissolving 10-40 mmol of urea in 40 ml of water, adding the mixedsolution of step (1) and stirring uniformly; (3) transferring the mixedsolution of step (2) into a reaction vessel and reacting at 120-160° C.for 6-12 hours; (4) carrying out suction filtration and water washing,followed by drying in a vacuum oven at 40-60° C.; and (5) calcining at350-500° C. for 2 to 4 hours in a muffle furnace.
 2. The method forpreparing the hollow molybdate composite microspheres according to claim1, wherein a ratio of a total mass of soluble nickel salt, cobalt saltand copper salt to a mass of molybdic acid in the step (1) is 1:1. 3.The method for preparing the hollow molybdate composite microspheresaccording to claim 2, wherein in the step (2), a stirring time is 0.5-1h.
 4. The method for preparing the hollow molybdate compositemicrospheres according to claim 3, wherein in the step (3), atemperature in the vacuum oven is 40-60° C.
 5. The method for preparingthe hollow molybdate composite microspheres according to claim 1,wherein in the step (2), a stirring time is 0.5-1 h.
 6. The method forpreparing the hollow molybdate composite microspheres according to claim5, wherein in the step (3), a temperature in the vacuum oven is 40-60°C.
 7. The method for preparing the hollow molybdate compositemicrospheres according to claim 1, wherein in the step (3), atemperature in the vacuum oven is 40-60° C.
 8. An application of thehollow molybdate composite microspheres prepared by the preparing methodof claim 1 as a catalyst for ammonia borane hydrolysis to producehydrogen.
 9. An application of the hollow molybdate compositemicrospheres prepared by the preparing method of claim 2 as a catalystfor ammonia borane hydrolysis to produce hydrogen.